In Universal Mobile Telecommunications System (UMTS)/High Speed Downlink Packet Access (HSDPA), several interference aware receivers have been specified for the User Equipment device (UE). Such receivers are termed “enhanced receivers” as opposed to the baseline receiver, which is typically a rake receiver. There are several different types of enhanced receivers in UMTS including an enhanced receiver type 1 having two branch receiver diversity, an enhanced receiver type 2 having a single branch equalizer, an enhanced receiver type 3 having two branch receiver diversity and an equalizer, and an enhanced receiver type 3i having two branch receiver diversity and inter-cell interference cancellation capability. The enhanced receivers can be used to improve performance, e.g., in terms of throughput and/or coverage.
In Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 10 (Rel-10), enhanced interference coordination techniques have been developed to mitigate potentially high interference, e.g., in a cell range expansion zone, while providing the UE with time-domain measurement restriction information. Further, for LTE Release 11 (Rel-11), advanced receivers based on Minimum Mean Square Error—Interference Rejection Combining (MMSE-IRC) with several covariance estimation techniques and interference-cancellation-capable receivers (for different types of signals and channels) have been studied. In the future, even more complex advanced receivers, e.g., Minimum Mean Square Error—Successive Interference Cancellation (MMSE-SIC), which is capable of performing nonlinear subtractive-type interference cancellation, can be used to further enhance system performance.
Such enhanced receiver techniques generally may benefit all deployments where relatively high interference of one or more signals is experienced when performing measurements on radio signals or channels transmitted by radio nodes or devices, but are particularly useful in heterogeneous deployments. However, these enhanced receiver techniques involve additional complexity, e.g., may require more processing power and/or more memory. Due to these factors, an advanced receiver may be used by the UE for mitigating interference only on specific signals or channels. For example, a UE may apply an interference mitigation or cancellation technique only on a data channel. In another example, a more sophisticated UE may apply interference mitigation on a data channel as well as on one or two common control signals. Examples of common control signals are reference signals, synchronization signals, etc.
It should be noted that the terms interference mitigation receiver, interference cancellation receiver, interference suppression receiver, interference rejection receiver, interference aware receiver, interference avoidance receiver, etc. are interchangeably used but they all belong to a category of an advanced receiver or an enhanced receiver. All of these different types of enhanced receivers improve performance by fully or partly eliminating the interference arising from at least one interfering source. The interfering source is generally the strongest interferer(s), which are signals from the neighboring cells when the action is performed in the UE. Therefore, a more generic term, “enhanced receiver,” which covers all variants of advanced receiver, is used hereinafter. Further, the corresponding interference handling techniques (e.g., interference cancellation, interference suppression, puncturing or interference rejection combining) for enhanced receivers are referred to herein as “enhanced receiver techniques.”
Co-channel, or intra-frequency, interference is the most common type of interference in the context of enhanced receivers. In a cellular communications network, a UE performs intra-frequency measurements, for example, in scenarios illustrated in FIG. 1. The intra-frequency measurements may be performed for various purposes such as, for example, Radio Resource Management (RRM), positioning, interference coordination, Self-Optimizing Network (SON), etc. More specifically, FIG. 1 illustrates transmission bandwidths for a UE, a current (or serving) cell of the UE, and a target cell for which the UE performs intra-frequency measurements for three different scenarios, namely, scenarios A, B, and C. The heights of the bars in each of the scenarios represent the corresponding transmission bandwidths. Note that while only one current (or serving) cell is illustrated for the UE, the UE may have multiple serving cells if Carrier Aggregation (CA) is used (i.e., one primary cell (pCell) and one or more secondary cells (sCells)). The transmissions in the current and target cells may be any one or more of: Downlink (DL) transmissions, Uplink (UL) transmissions, and device-to-device transmissions. For example, in a Time Division Duplexing (TDD) band, DL and UL transmissions occur on the same frequency according to an UL/DL configuration. In earlier LTE releases, TDD networks have been assumed to always be synchronized, and TDD cells have been assumed to have the same UL/DL configuration. However, e.g., with dynamic TDD, it may become possible to use different UL/DL configurations in different cells (currently under study in 3GPP), which implies the possibility of a scenario where DL is transmitted in one cell while UL is transmitted in another cell at the same time. Similarly, device-to-device transmission may occur in parallel to another device-to-device transmission or another DL or UL transmission. Note that while intra-frequency interference is the most common in the context of enhanced receivers, enhanced receivers may also potentially handle inter-frequency or even inter-Radio Access Technology (RAT) interference, e.g., when the interfering channel transmission bandwidth overlaps with the measured bandwidth.
Enhanced receivers may be used in either homogeneous cellular communications networks (i.e., homogeneous deployments) or heterogeneous cellular communications networks (i.e., heterogeneous deployments). Note that there may also be scenarios where part(s) of the cellular communications network have a heterogeneous deployment(s) and other part(s) have a homogeneous deployment(s). This is referred to as a mixed deployment.
Homogeneous cellular communications networks are typically deployments with the same/similar type of radio network nodes and/or similar coverage and cell sizes and inter-site distances. Although interference coordination may be more challenging in heterogeneous deployments, there may also be significant benefits of using enhanced receivers in homogeneous cellular communications networks.
In contrast to homogeneous network deployments, heterogeneous network deployments utilize low-power nodes (such as pico base stations, home Evolved Node Bs (eNBs), relays, remote radio heads, etc.) for enhancing performance of the macro network in terms of the network coverage, capacity, and service experience of individual users. The interest in heterogeneous network deployments has been constantly increasing over the last few years. At the same time, there has been realized a need for enhanced interference management techniques to address new interference issues caused by, for example, a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks.
In 3GPP, heterogeneous network deployments have been defined as deployments where low-power nodes (e.g., pico nodes) of different transmit powers are placed throughout a macro cell layout, which implies non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adapt to traffic needs and the environment. However, heterogeneous network deployments also bring challenges for which the network has to be prepared in order to ensure efficient network operation and superior user experience. Some challenges are related to increased interference resulting from the attempt to increase coverage areas of small cells associated with low-power nodes (i.e., cell range expansion). Other challenges are related to potentially high interference in the UL due to a mix of large and small cells.
According to 3GPP, heterogeneous deployments consist of deployments where low power nodes are placed throughout a macro cell layout. The interference characteristics in a heterogeneous deployment can be significantly different than in a homogeneous deployment, in DL or UL or both. In this regard, FIG. 2 illustrates one example of a heterogeneous cellular communications network 10 that includes a macro base station 12 (e.g., an eNB) serving a macro cell 14, a number of low-power base stations 16-1 through 16-3 (e.g., home eNBs) serving corresponding Closed Subscriber Group (CSG) cells 18-1 through 18-3, and a low-power base station 20 (e.g., a pico base station) serving a pico cell 22. FIG. 2 also illustrates four cases, each illustrating a different interference scenario. In case (A), a macro UE 24 (i.e., a UE served by the macro base station 12) with no access to the CSG cell 18-1 will experience interference caused by transmissions from the low-power base station 16-1 serving the CSG cell 18-1. In case (B), a macro UE 26 with no access to the CSG cell 18-2 causes severe interference towards the low-power base station 16-2 serving the CSG cell 18-2. In case (C), a CSG UE 28 (i.e., a UE served by a CSG cell) is served by the CSG cell 18-3 but will experience interference from the low-power base station 16-2 serving the CSG cell 18-2. Lastly, in case (D), a UE 30 is served by the low-power base station 20 in an expended cell range area 32 of the pico cell 22. The UE 30 in the extended cell range area 32 of the pico cell 22 will experience DL interference from the macro base station 12 and may also cause UL interference to the macro base station 12. Note that while several of the cases illustrated in FIG. 2 use CSG cells, a heterogeneous deployment does not necessarily involve CSG cells.
To ensure reliable and high-bitrate transmissions as well as robust control channel performance, maintaining a good signal quality is a must in cellular communications networks. The signal quality is determined by the received signal strength and its relation to the total interference and noise received by the receiver. A good network plan, which among others things includes cell planning, is a prerequisite for successful network operation. However, the network plan is static. For more efficient radio resource utilization, a good network plan has to be complemented at least by semi-static and dynamic radio resource management mechanisms, which are also intended to facilitate interference management, and deploying more advanced antenna technologies and algorithms.
One way to handle DL interference is, for example, to adopt enhanced receiver technologies, e.g. by implementing interference cancellation mechanisms in UEs. Another way, which can be complementary to the former, is to design efficient interference coordination algorithms and transmission schemes in the network. The coordination may be realized in static, semi-static, or dynamic fashion. Static or semi-static schemes may rely on reserving time-frequency resources (e.g., a part of the bandwidth and/or time instances) that are orthogonal for strongly interfering transmissions. Dynamic coordination may be implemented, e.g., by means of scheduling. Such interference coordination may be implemented for all or specific channels (e.g., data channels or control channels) or signals.
Specifically for heterogeneous network deployments, enhanced Inter-Cell Interference Coordination (eICIC) mechanisms for ensuring that the UE performs at least some measurements (e.g., Radio Resource Management (RRM), Radio Link Monitoring (RLM), and Channel State Information (CSI) measurements) in low-interference subframes of the interfering cell have been standardized. These mechanisms involve configuring patterns of low-interference subframes at transmitting nodes to thereby reduce interference, and configuring measurement patterns for UEs to thereby indicate to the UEs low-interference measurement occasions.
Two types of patterns have been defined for eICIC in LTE Rel-10 to enable restricted measurements in DL, namely, restricted measurement patterns and transmission patterns. The restricted measurement patterns are configured by a network node and signaled to the UE. The transmission patterns, which are also known as Almost Blank Subframe (ABS) patterns, are configured by a network node and describe the transmission activity of a radio node. Transmission patterns may be exchanged between radio nodes.
More specifically, with regard to restricted measurement patterns for the DL in LTE, the UE may receive a set of restricted measurement patterns to enable measurements for RRM (e.g., Reference Signal Received Power (RSRP)/Reference Signal Received Quality (RSRQ)), RLM, and CSI as well as for demodulation. As defined in 3GPP Technical Specification (TS) 36.331 V10.1.0, the UE may receive, via Radio Resource Control (RRC) UE-specific signaling, the following set of patterns:                Pattern 1: A single RRM/RLM measurement resource restriction for the serving cell.        Pattern 2: One RRM measurement resource restriction for neighbor cells (up to 32 cells) per frequency (currently only for the serving frequency).        Pattern 3: Resource restriction for CSI measurement of the serving cell with two subframe subsets configured per UE.A pattern is a bit string indicating restricted and unrestricted subframes characterized by a length and periodicity, which are different for Frequency Division Duplexing (FDD) and TDD (40 subframes for FDD and 20, 60, or 70 subframes for TDD). The restricted subframes are configured to allow the UE to perform measurements in subframes with improved interference conditions, which may be implemented by configuring ABS patterns at the base stations.        
In addition to RRM/RLM, Pattern 1 may also be used to enable UE Receive (Rx)—Transmit (Tx) measurements, which are timing measurements similar to round trip time, in low-interference conditions or in principle for any Cell-Specific Reference Signal (CRS) based measurement to improve the measurement performance when strong interference may be reduced by configuring low-interference subframes. Pattern 3 would typically be used for enhancing channel quality reporting and improving the performance of channel demodulation and decoding (e.g., of data channels such as Physical Downlink Shared Channel (PDSCH), control channels such as Physical Downlink Control Channel (PDCCH), Physical Control Format Indicator Channel (PCFICH), and Physical Hybrid Automatic Repeat Request (ARQ) Indicator Channel (PHICH)). Pattern 1 and Pattern 2 may also be used for enabling low-interference conditions for common signals (e.g., Primary Synchronization Signal (PSS)/Secondary Synchronization Signal (SSS)), common channels, and broadcast/multicast channels (e.g., Physical Broadcast Channel (PBCH)), when the strong interference can be reduced or avoided (e.g., when a time shift is applied to ensure that the common channels/signals are interfered by data whose interference may be avoided by configuring low-interference subframes and thereby suppressing the interfering data transmissions).
With regard to DL ABS patterns, an ABS pattern indicates subframes when the base station (e.g., eNB) restricts its transmissions (e.g., does not schedule or transmits at a lower power). The subframes with restricted transmissions are referred to as ABS subframes. In the current LTE standard, a base station can suppress data transmissions in ABS subframes but the ABS subframes cannot be fully blank—at least some of the control channels and physical signals are still transmitted. Examples of control channels that are transmitted in ABS subframes even when no data is transmitted are PBCH and PHICH. Examples of physical signals that have to be transmitted regardless of whether the subframes are ABSs or not are CRS and synchronization signals (PSS and SSS). Positioning Reference Signals (PRS) may also be transmitted in ABS subframes. If a Multimedia Broadcast Single Frequency Network (MBSFN) subframe coincides with an ABS, the MBSFN subframe is also considered as an ABS, as set forth in 3GPP TS 36.423. CRS are not transmitted in MBSFN subframes, except for the first symbol, which allows for avoiding CRS interference from an aggressor cell to the data region of a measured cell. ABS patterns may be exchanged between base stations, e.g., via X2, but these patterns are not signaled to the UE.
In LTE Rel-11, for enhanced receivers (e.g., capable of interference cancellation), the information about a strongly interfering cell, which is also known as an aggressor cell, may be provided to facilitate handling the strong interference generated by transmissions in that cell. The currently agreed information is as below, i.e., the following information about the interfering cells may be provided to the UE: Physical Cell Identity (PCI), number of CRS antenna ports, and MBSFN subframe configuration.
NeighCellsCRS-Info-r11 ::= CHOICE {releaseNULL,setupCRS-AssistanceInfoList-r11}CRS-AssistanceInfoList-r11 ::= SEQUENCE (SIZE (1.. maxCellReport))OF CRS-AssistanceInfoCRS-AssistanceInfo ::= SEQUENCE {physCellId-r11PhysCellId,antennaPortsCount-r11ENUMERATED {an1, an2, an4,spare1},mbsfn-SubframeConfigList-r11MBSFN-SubframeConfigList}
In high interference scenarios, it may be challenging to read System Information (SI) including the Master Information Block (MIB), which is transmitted via PBCH, and the System Information Blocks (SIBs), which are transmitted via PDSCH. Hence, some UEs are likely to have interference cancellation capability to acquire PBCH while performing interference cancellation of the aggressor cell interference, e.g., in a radio frame aligned scenario such as that illustrated in FIG. 3 where PBCH transmissions by an aggressor cell cause interference to PBCH transmissions by a victim cell, which may be a serving cell or a measured cell. MIB interference cancellation may or may not involve MIB decoding.
The MIB is mapped on the Broadcast Control Channel (BCCH) and carried on the Broadcast Channel (BCH) while all other SI messages are mapped on the BCCH and dynamically carried on the Downlink Shared Channel (DL-SCH) where they can be identified through the System Information Radio Network Temporary Identifier (SI-RNTI). The MIB is transmitted according to a fixed schedule with a periodicity of 40 microseconds (ms) in subframes #0. To improve MIB detection performance, three redundancy versions are also signaled with a 10 ms period.
The SIB Type 1 (SIB1) is transmitted with a periodicity of 80 ms and repetitions made within 80 ms. The first transmission of SIB1 is scheduled in subframe #5 of radio frames for which the System Frame Number (SFN) mod 8=0, and repetitions are scheduled in subframe #5 of all other radio frames for which SFN mod 2=0, i.e., with a 20 ms period. The scheduling of other SI messages (e.g., periodicity and SI-window) is flexible and indicated by SIB1. Each SIB is contained only in a single SI message. Only SIBs having the same scheduling requirement (periodicity) can be mapped to the same SI message. There is also a limit on the maximum size of a SI message (217 bytes with Downlink Control Information (DCI) format 1C and 277 bytes with 1a format). The obtained SI is stored by the UE and considered invalid after three hours. The paging message is used to inform UEs in RRC_IDLE and UEs in RRC_CONNECTED about a system information change.
System information may also be provided to the UE by means of dedicated signaling, e.g. upon handover. Furthermore, to facilitate receiver performance in high interference conditions, according to 3GPP TS 36.300, the network may provide SIB1 to the UE in the Cell Range Expansion (CRE) region by a dedicated RRC signaling to assist UE system information acquisition. According to 3GPP TS 36.331, in addition to system information broadcast, the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) may provide the same SIB1 via dedicated signaling in the RRCConnectionReconfiguration message, as follows:
RRCConnectionReconfiguration-v1020-IEs ::= SEQUENCE {sCellToReleaseList-r10SCellToReleaseList-r10OPTIONAL, --Need ONsCellToAddModList-r10SCellToAddModList-r10OPTIONAL, --Need ONnonCritical ExtensionRRCConnectionReconfiguration-v11xx-IEs OPTIONAL}RRCConnectionReconfiguration-v11xx-IEs ::= SEQUENCE {systemInfomationBlockType1Dedicated-r11OCTET STRING(CONTAININGSystemInformationBlockType1)OPTIONAL,-- Need ONnonCritical ExtensionSEQUENCE { }OPTIONAL-- Need OP}
The LTE standards starting with LTE Rel-9 allow the UE to create autonomous gaps for intra-frequency, inter-frequency, or inter-RAT Cell Global Identification (CGI)/SI reading. The target cell whose CGI can be read can be an intra-frequency cell, an inter-frequency cell, or even an inter-RAT cell (e.g., Universal Terrestrial Radio Access Network (UTRAN), Global System for Mobile Communications (GSM) EDGE Radio Access Network (GERAN), Code Division Multiple Access 2000 (CDMA2000) or High Rate Packet Data (HRPD)). There are at least a few well known scenarios for which the serving cell may request the UE to report the CGI of the target cell, namely, verification of a CSG cell, establishment of Self Organizing Network (SON) Automatic Neighbor Relation (ANR), and Minimization of Drive Tests (MDT). In addition to the CGI, the SI also contains other information such as, e.g., SFN, which may be acquired for many purposes, e.g., for positioning when SFN of the reference cell is not known (e.g., inter-frequency Reference Signal Time Difference (RSTD) measurements when the reference cell and the neighbor cells in the assistance data are not on the serving cell frequency).
In LTE, the UE is required to report the intra-frequency E-UTRAN CGI (ECGI) from a target intra-frequency cell within about 150 ms provided that the Signal-to-Interference-Plus-Noise Ratio (SINR) for the target intra-frequency cell is at a certain level or higher. In order to meet this requirement, the UE is allowed to create autonomous gaps in the DL and UL during which to read the ECGI of the target intra-frequency cell. Under continuous allocation, the UE is required to transmit a certain number of Acknowledgements (ACKs)/Negative Acknowledgements (NACKs) on the UL to ensure that the UE does not create excessive gaps. In UTRAN, the target cell's CGI acquisition is much longer, e.g., more than one second, depending upon the periodicity of SIB Type 3 (SIB3), which contains the CGI. Furthermore due to the autonomous gaps created by the UE to acquire the CGI of the target cell, the interruption of the data transmission and reception from the serving cell can be 600 ms or longer.
As discussed above, in E-UTRAN, the serving cell can request the UE to acquire the CGI, which uniquely identifies a cell, of the target cell. In order to acquire the CGI of the target cell, the UE has to read at least part of the SI of the target cell including the MIB and the relevant SIB. The reading of the SI for the acquisition of the CGI of the target cell is carried out during measurement gaps in the transmission of the UL to the serving cell and/or the reception of the DL from the serving cell that are autonomously created by the UE. In LTE, the UE reads the MIB and SIB1 of the target cell to acquire its CGI (i.e., ECGI when the target cell is E-UTRAN intra- or inter-frequency).
In LTE, the MIB includes a limited number of most essential and most frequently transmitted parameters that are needed to acquire other information from the cell, and is transmitted on the BCH. In particular, the following information is currently included in MIB: DL bandwidth, PHICH configuration, and SFN. The LTE SIB1, as well as other SIB messages, is transmitted on DL-SCH. In LTE, the SIB1 contains, e.g., the following information: Public Land Mobile Network (PLMN) identity, cell identity, which can be a physical cell ID (PCI) or a cell global ID (CGI), CSG identity and indication, frequency band indicator, SI window length, and scheduling information for other SIBs. The LTE SIB1 may also indicate whether a change has occurred in the SI messages. The UE is notified about upcoming changes in the SI by paging messages (i.e., upon receipt of a paging message, the UE knows that the SI will change at the next modification period boundary). The modification period boundaries are defined by SFN values for which SFN mod m=0, where m is the number of radio frames comprising the modification period. The modification period is configured by system information. In case of inter-RAT UTRAN, the UE reads the MIB and SIB3 of the target UTRAN cell to acquire the CGI of the target UTRAN cell.
In LTE, a timer T321 is used when a CGI report is requested by the network. The T321 timer is started upon receiving a measConfig message including a reportConfig with the purpose set to reportCGI. The timer T321 is stopped upon acquiring the information needed to set all fields of cellGlobalId for the requested cell or upon receiving measConfig that includes removal of the reportConfig with the purpose set to reportCGI. Upon expiry of the timer T321, the measurement reporting procedure is initiated and the UE stops performing the related measurements and removes the corresponding measId.